Silver halide photographic materials have hitherto been demanded to have a high sensitivity. In particular, spectrally sensitized silver halide photographic materials have keenly demanded to have a high sensitivity.
The spectral sensitization is a very important and essential technique in the production of a light-sensitive material having a high sensitivity and an excellent color reproducibility. The spectral sensitizer has a function to absorb light in a long wavelength region where the silver halide photographic emulsion inherently does not absorb light in a substantial sense and to transmit the light absorption energy to the silver halide. The increased amount of light captured by the spectral sensitizer is advantageous to enhance the photographic sensitivity. Accordingly, it has been attempted to increase the captured light amount by increasing the amount of the spectral sensitizer added to a silver halide emulsion. However, if the spectral sensitizer is added to a silver halide emulsion in an amount exceeding the optimum range, the sensitivity is, on the contrary, greatly reduced. This is generally called as dye desensitization and is a phenomenon that desensitization occurs in the region to which the silver halide is inherently light-sensitive and where the sensitizing dye absorbs substantially no light. If the dye desensitization is large, the spectral sensitization effect may be provided but overall sensitivity is reduced. In other words, with the reduction in dye desensitization, the sensitivity in the region where the sensitizing dye absorbs light (namely, spectral sensitivity) increases correspondingly. Therefore, the improvement in dye desensitization is a great subject in the spectral sensitization technique. The dye desensitization is larger as the sensitizing dye has its light-sensitive region at a longer wavelength. This is described in T. H. James, The Theory of the Photographic Process, pp. 265-268 (Macmillan, 1966).
Further, as described in T. Tani et al, Journal of the Physical Chemistry, Vol. 94, p. 1298 (1990), sensitizing dyes having a reduction potential more noble than -1.25 V are known to provide a low relative quantum yield in spectral sensitization. In order to increase the relative quantum yield of such a dye in spectral sensitization, supersensitization by capturing positive holes has been proposed as described in the above-described The Theory of the Photographic Process, pp. 259-265 (1966).
In order to solve the above-described desensitization, JP-A-5-216152 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describes hydrazine compounds having a specific structure.
However, still higher sensitivity and more improved storage stability have been demanded.